Method for manufacturing semiconductor device

ABSTRACT

Included herein are, a step of forming an active region for a semiconductor device on a front surface of a SiC substrate, a step of forming a SiC substrate-to-drain electrode bonding region on a back surface of the SiC substrate by grinding it using an abrasive whose average abrasive grain size is within a specified range, a step of depositing a film of a first drain electrode on the SiC substrate-to-drain electrode bonding region, a step of electrically connecting the first drain electrode with the SiC substrate-to-drain electrode bonding region, and a step of depositing a film of a second drain electrode on the first drain electrode, so that a SiC semiconductor device having a high mechanical strength with a reduced energization loss is achieved.

TECHNICAL FIELD

The present application relates to a method for manufacturing asemiconductor device and, in particular, relates to a method formanufacturing a front-to-back conductive semiconductor device.

BACKGROUND ART

Semiconductor devices that use silicon carbide (SiC) semiconductorsubstrates in order to allow the semiconductor devices to be made, forexample, higher in breakdown voltage, lower in loss and usable in a hightemperature environment, and that are superior in withstand voltage andheat resistance to silicon (Si) semiconductor devices, are applied topower semiconductor devices such as a MOSFET (metal-oxide-semiconductorfield-effect transistor), a schottky barrier diode and the like. Forexample, in the case of a SiC semiconductor MOSFET with a breakdownvoltage class of 1 to 1.2 kV, there is provided an ON-resistance of 5 mΩcm² or less, the resistance value of which is half or less when comparedwith a Si semiconductor MOSFET or IGBT (Insulated Gate BipolarTransistor) with the same breakdown voltage. The reason why the use of aSiC semiconductor can largely reduce the ON resistance in comparisonwith a Si semiconductor, is that the SiC semiconductor has a highdielectric-breakdown electric field and allows a voltage-withstandinglayer (drift layer) for achieving the same breakdown voltage to bethinner than that of the Si semiconductor, and further allows the dopingamount of impurities for the voltage withstand layer to be higher, andsomething like that. It is thought that, hereafter, replacement of mostof Si semiconductor IGBTs by these devices as inverter components willbe facilitated with achievement of: improvement in terms of theirmanufacturing cost; enhancement in their process technology; and otherenhancement in their capabilities.

Heretofore, as a method of establishing bonding between a semiconductorsubstrate and an electrode at the time of manufacturing a front-to-backconductive semiconductor device, such a method is proposed thatcomprises a step of forming a semiconductor element structure on themajor surface side of a silicon semiconductor substrate and thereafter,in the last process, grinding the back-surface side of the siliconsemiconductor substrate and ion-implanting therein impurities whoseconductivity type is the same as that of the back surface, and thenforming thereon a metal thin film to be provided as an electrode (see,for example, Patent Document 1).

Meanwhile, as a method of establishing assured bonding while reducingpower loss at the time of forming an electrode onto a silicon nitride(SiC) semiconductor substrate, such a method is proposed in which, withrespect to a semiconductor element provided with the SiC substrate, aheat treatment is locally applied by optical heating to the electrode onthe back surface of the substrate and at that treatment, its procedureand the conditions for the heat treatment are optimized, so that thesemiconductor element can be manufactured in a good yield (see, forexample, Patent Document 2).

CITATION LIST Patent Document

Patent Document 1: Japanese Patent Application Laid-open No. H09-008062(Paragraph 0007, FIG. 1)

Patent Document 2: Japanese Patent Application Laid-open No. 2010-186991(Paragraph 0071, FIG. 1)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

With respect to a front-to-back conductive semiconductor device, inorder to reduce the energization loss of the semiconductor device, it iseffective to cause thinning of its semiconductor substrate that acts asan electrically resistive component at the time of energization.However, because the contact area between the semiconductor substrateand the electrode changes depending on the degree of ground-surfaceunevenness that occurs when the thinning of the semiconductor substrateis performed, there is a problem that the electrical resistance betweenthe semiconductor substrate and the electrode varies. Further, there isa problem that the chip strength is reduced due to crystal defectsinside the semiconductor substrate that occur during grinding processingat the time of performing the thinning.

In Patent Document 1, there is described that, in the last process formanufacturing the semiconductor device, grinding processing is executedand thereafter, the metal thin film is formed after the implantation ofthe impurities; however, in order to establish electrical connectionbetween the semiconductor substrate and the metal thin film, it isrequired to form their bonding plane at the interface between thesemiconductor substrate and the metal thin film by applying thermal oroptical energy or the like thereto. According to the structure of PatentDocument 1, there is a problem that it is unclear whether adequatebonding has been established between the semiconductor substrate and themetal thin film.

According to Patent Document 2, the bonding between the semiconductorsubstrate and the electrode is achieved in such a manner that theelectrode is formed after the thinning of the semiconductor substrateand then the heat treatment is applied from the back-surface side byusing a high-power optical heating method.

However, because the electrical resistance between the semiconductorsubstrate and the metal thin film increases due to inclusion of aforeign material at the time of grinding the semiconductor substrate bymachining, and the chip strength decreases due to crystal defects thatoccur during grinding of the semiconductor substrate, there is a problemthat the chip is broken when it is mounted on a module board or when aload is applied thereto through energization.

This application has been made to solve the problems as described above,and an object thereof is to provide, with respect to a front-to-backconductive semiconductor device, a method for manufacturing a highquality semiconductor device, that can stably accomplish excellentbonding and low-resistance conduction between the semiconductorsubstrate and the electrode even though thinning of the semiconductorsubstrate has been performed in order to reduce the energization loss,and that can suppress the mechanical strength of the semiconductordevice from decreasing due to grinding for the thinning.

Means for Solving the Problems

A method for manufacturing a semiconductor device according to theapplication is characterized by comprising: a first grinding step ofgrinding, using an abrasive, a back-surface side of a SiC substratehaving an active region formed on a front-surface side thereof; a secondgrinding step of grinding a back-surface side of the SiC substrateproviding after the first grinding step, using an abrasive having anaverage abrasive grain size of not less than 1 μm and not more than 5μm; a step of depositing a film of a first main electrode on anelectrode bonding region formed by the second grinding step; a step ofelectrically connecting the first main electrode with the electrodebonding region by using laser irradiation; and a step of depositing afilm of a second main electrode on the first main-electrode subjected tothe laser irradiation, wherein a grinding amount of the SiC substratefor grinding the back-surface side by the first grinding step and thesecond grinding step is 250 μm or more; and wherein a thickness of theSiC substrate becomes not less than 50 μm and not more than 150 μm.

Effect of the Invention

According to the application, the substrate is ground by using thegrinding abrasive having a controlled abrasive grain size, so that it ispossible not only to establish excellent bonding but also to achieve asemiconductor device having a high mechanical strength with a reducedenergization loss.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view for illustrating a configuration of a mainpart of a semiconductor device manufactured by the semiconductor devicemanufacturing method according to Embodiment 1 of the application.

FIG. 2 is a top view for illustrating a configuration of a main part ofthe semiconductor device manufactured by the semiconductor devicemanufacturing method according to Embodiment 1 of the application.

FIG. 3 is a flowchart showing manufacturing steps in the semiconductordevice manufacturing method according to Embodiment 1 of theapplication.

FIG. 4A through FIG. 4J are sectional views of a main part of asemiconductor device at respective manufacturing steps by thesemiconductor device manufacturing method according to Embodiment 1 ofthe application.

FIG. 5 is atop view of a manufacturing apparatus used in thesemiconductor device manufacturing method according to Embodiment 1 ofthe application.

FIG. 6A and FIG. 6B are an enlarged sectional view and a top view of amain part of the manufacturing apparatus used in the semiconductordevice manufacturing method according to Embodiment 1 of theapplication.

FIG. 7A and FIG. 7B are graphs showing relationships between an averageabrasive grain size of an abrasive and an ON-resistance and between thatsize and a ball transverse rupture strength, with respect to thesemiconductor device manufacturing method according to Embodiment 1 ofthe application.

FIG. 8A and FIG. 8B are diagrams for illustrating how to measure thegrain size of an abrasive, with respect to the semiconductor devicemanufacturing method according to Embodiment 1 of the application.

FIG. 9 is a sectional view for illustrating a configuration of a mainpart of a semiconductor device manufactured by the semiconductor devicemanufacturing method according to Embodiment 2 of the application.

FIG. 10 is a flowchart showing manufacturing steps in the semiconductordevice manufacturing method according to Embodiment 2 of theapplication.

FIG. 11A, FIG. 11B and FIG. 11C are sectional views of a main part of asemiconductor device at respective manufacturing steps by thesemiconductor device manufacturing method according to Embodiment 2 ofthe application.

FIG. 12A and FIG. 12B are graphs showing relationships between anaverage abrasive grain size of an abrasive and an ON-resistance andbetween that size and a cracking ratio at the time of irradiation, withrespect to the semiconductor device manufacturing method according toEmbodiment 2 of the application.

MODES FOR CARRYING OUT THE INVENTION Embodiment 1

FIG. 1 is a sectional view showing a cell structure of a main part of asemiconductor device manufactured by the semiconductor devicemanufacturing method according to Embodiment 1 of the application. Acell structure 101 is provided as a planer gate structure of aSiC-MOSFET that is a SiC semiconductor device. As shown in FIG. 1, inthe cell structure 101, a first drift layer 2 made of n-type SiC isformed as a first-layered epitaxial film on a first major surface(hereinafter, referred to as a front surface) that is placed on theupper side of an n-type SiC substrate 1, and a second drift layer 3 isformed as a second-layered epitaxial film on the surface of the firstdrift layer 2. On the surface of the second drift layer 3, a pair ofactivated p-type base regions 4 a is formed and further, an activatedn-type source region 5 a is formed partly on each of the surfaces of thepair of activated p-type base regions 4 a. In addition, a gate electrode7 is formed so that it is covered with a gate insulating film 6 and sothat the pair of base regions 4 a and the pair of source regions 5 a areplaced below the both end portions of said electrode 7 and a part of thesecond drift layer 3 located between the pair of base regions 4 a isplaced just beneath the center of the gate electrode 7. In this manner,on the front surface of the SiC substrate 1, an active region is formedthat comprises the first drift layer 2, the second drift layer 3, thebase regions 4 a, the source regions 5 a, the gate insulating film 6 andthe gate electrode 7. Moreover, a source electrode 8 is formed so as tocover the pair of source regions 5 a and to further cover the gateelectrode 7 covered with the gate insulating film 6. Meanwhile, on asecond major surface (hereinafter, referred to as a back surface) thatis placed on the lower side of the SiC substrate 1, a drain electrode 10as a first main electrode is formed on a SiC substrate-to-drainelectrode bonding region 9 that is formed on the back-surface side ofthe SiC substrate 1 having been ground away to the extent correspondingto a specified thickness thereof. Note that, actually, the wholeconfiguration of the SiC semiconductor device is such a configuration inwhich cell configurations each shown in FIG. 1 are successively placedacross their both ends each as a line-symmetrical axis.

FIG. 2 is a top view corresponding to FIG. 1. As shown in FIG. 2, thep-type base regions 4 a and the n-type source regions 5 a are formed inthe second drift layer 3. The gate insulating film 6 shown in FIG. 2that encloses the gate electrode 7 is placed in a grid-like manner onthe upper side of the second drift layer 3. Although the sourceelectrode 8 is further placed on the second drift layer 3 and the gateinsulating film 6, it is omitted from illustration in FIG. 2. The crosssection along a broken line portion A shown in FIG. 2 is what is shownin FIG. 1. Note that, where appropriate, the cell configuration may bemodified according to a desired characteristic.

In the following, the semiconductor device manufacturing methodaccording to Embodiment 1 of the application will be described based onFIG. 3. FIG. 3 is a flowchart showing manufacturing steps in thesemiconductor device manufacturing method according to Embodiment 1 ofthe application, and a SiC semiconductor device is manufacturedaccording to this flow. FIG. 4A through FIG. 4J are sectional views of acell at respective steps of manufacturing the semiconductor deviceaccording to Embodiment 1.

First of all, in preparatory processing for the SiC substrate (StepS301), as shown in FIG. 4A, the first drift layer 2 made of n-type SiCis formed as a first-layered epitaxial film on the front surface of then-type SiC substrate 1 and thereafter, the second drift layer 3 isformed as a second-layered epitaxial film in a contacting manner withthe upper surface of the first drift layer 2 by using an epitaxialcrystal growth method at a growth temperature that is lower than thegrowth temperature of the first drift layer 2.

Subsequently, in base-region forming processing (Step S302), after theabove-described epitaxial crystal growth was performed, impurities areion-implanted into portions in a top layer of the second drift layer 3that are apart from each other with a specified interval therebetween,after the formation of a mask pattern made of a resist or the like, tothereby form the pair of p-type base regions 4. FIG. 4B shows a verticalsection structure of the element after the removal of the mask patternmade of a resist or the like. Examples of the impurity whoseconductivity type is developed as p-type in the second drift layer 3include, for example, boron (B) and aluminum (Al).

Then, in source-region forming processing (Step S303), impurities areion-implanted into the respective p-type base regions 4 after theformation of a mask pattern made of a resist or the like, to therebyform the n-type source regions 5. FIG. 4C shows a vertical sectionstructure of the element after the removal of the mask pattern. Examplesof the n-type impurity include, for example, phosphorous (P), nitrogen(N) and arsenic (As).

Subsequently, in activation processing for the base and source regions(Step S304), after the above described ion-implantation was executed,the SiC wafer is heat treated at a high temperature using a heattreatment apparatus, so that the p-type and n-type ions implanted in thebase regions 4 and the source regions 5 are electrically activated, tothereby provide the activated base regions 4 a and source regions 5 a.FIG. 4D shows a vertical section structure of the element after thatheat treatment.

Then, in gate-insulating film forming processing (Step S305), as shownin FIG. 4E, a gate insulating film 6 a is formed on the second driftlayer 3 having the activated base regions 4 a and source regions 5 a, byusing a thermal oxidation method or a deposition method such as achemical vapor deposition or the like. Subsequently, in gate electrodeforming processing (Step S306), a film of the gate electrode 7 isdeposited on the insulating film 6 a and then patterning of the gateelectrode 7 is performed as shown in FIG. 4F. The gate electrode 7 ispatterned so that the pair of base regions 4 a and the pair of sourceregions 5 a are placed below the both end portions of said electrode 7and a part of the second drift layer 3 located between the pair of baseregions 4 a is placed just beneath the center of said electrode 7.Furthermore, as shown in FIG. 4G, a residual portion of the insulatingfilm 6 a on each of the source regions 5 a is removed using aphotolithographic technique and an etching technique.

Thereafter, in interlayer-insulating film forming processing (StepS307), as shown in FIG. 4H, a deposition of an interlayer insulatingfilm 6 b and its patterning are performed.

Then, in SiC substrate thinning processing (Step S308), as shown in FIG.4I, thinning of the substrate is performed from the back-surface side insuch a manner that machining is applied using a grinding abrasive madeup of aluminum abrasive grains or diamond abrasive grains, to the backsurface of the SiC substrate 1, thereby to make the SiC substrate 1 thinand further to form, on the back-surface side of the substrate, the SiCsubstrate-to-drain electrode bonding region 9 in which unevenness orcrystal strain due to grinding is introduced for establishing bondingwith a back-surface side electrode. By that thinning, when the thicknessof the SiC substrate is 150 μm or less, the influence of the transverserupture strength will appear remarkably and thus the effect of theapplication is more remarkable than otherwise.

FIG. 5 is atop view of a back grinding apparatus of an infeed grindingsystem for thinning the SiC substrate 1. As shown in FIG. 5, a wafer 12set in a wafer cassette 11 is transported by a transport robot 13 to analignment mechanism 14. Thereafter, it is moved using a transport arm 15to a wafer delivery section 17. Thereafter, it is moved to the positionof a first grinding stage 18 in such a manner that a grinding processingstage 16 is rotated counterclockwise. It is moved, after completion ofwafer grinding up to a preset thickness at the first grinding stage, tothe position of a second grinding stage 19 in such a manner that thegrinding processing stage 16 is further rotated counterclockwise.

At the second grinding stage, in a manner similar to that at the firstgrinding stage, processing of grinding a specified amount is performed.

It is noted that, as a method of grinding processing on the back-surfaceside, infeed grinding may be applied in which, as represented by asectional perspective view in FIG. 6A, after a surface protective member23 made of a polyester protective member and an acrylic adhesive pastewas affixed to the front-surface side of the wafer 22, saidfront-surface side is sucked to a suction stage 24 and thereafter, agrinding wheel 26 on which a segmental grinding abrasive 27 is fixed ismoved closer onto the wafer at a constant speed while the suction stage24 is being rotated in a fixed direction and while the grinding wheel 26is being rotated in a fixed direction. FIG. 6B is a diagram showing apositional relationship between the wafer and the grinding abrasive inFIG. 6A, viewed from the upper side thereof. Instead, creep feedgrinding may be carefully applied in which a workpiece is moved in afixed direction against an abrasive that is rotating. On the grindingwheel 26, a contact-type thickness meter 28 is provided.

Then, in order to get bonding with the SiC substrate through the SiCsubstrate-to-drain electrode bonding region 9, in drain electrodeforming processing (Step S309), the drain electrode 10 is formed on theback-surface side by using nickel, vanadium, aluminum, an aluminum alloymade of aluminum and silicon, or the like. Subsequently, in SiCsubstrate-to-drain electrode bonding processing (Step S310), as shown inFIG. 4J, electrical bonding between the SiC substrate and the drainelectrode is carried out in such a manner that the wafer is heated usingan anneal furnace that uses heater-based heating or the like or ananneal furnace that uses lamp-based heating or the like.

Lastly, in source electrode forming processing (Step S311), after thefront-surface side of the SiC semiconductor device was cleanedbeforehand using, on the front-surface side of the wafer, an aqueoussolution containing hydrofluoric acid or an aqueous solution containingammonia and hydrogen peroxide water, a metal barrier made of titanium ora titanium compound such as titanium nitride (TiN) or the like is formedon the source regions 5 a and the interlayer insulating film 6 b, andthereafter, a film of the source electrode 8 made of aluminum, analuminum alloy of aluminum and silicon, nickel or the like is depositedand subjected to patterning, so that the cell structure 101 of the SiCsemiconductor device shown in FIG. 1 is completed.

The material to be used for the source electrode 8 may be selected asappropriate according to a bonding method on the front-surface side,such as wiring, soldering or the like.

It is noted that, when nickel is used for the source electrode 8 and thedrain electrode 10, the bonding state at the time the chip is bondedwill be degraded because the wettability between the solder alloy andnickel becomes poor due to oxidation of its surface, so that, on thesurface of nickel, a metal that is outwardly less reactive than that,such as gold, silver or the like, may be used as a protective film.

In FIG. 7A, there is shown a relationship at the time the thinningprocessing is executed by the infeed grinding system as shown in FIG. 5,FIG. 6A and FIG. 6B that uses diamond abrasive grains, between anaverage abrasive grain size of the abrasive used in the last grinding,namely, at the second grinding stage 19, and a normalized ON-resistanceof the cell structure 101 of the obtained SiC-MOSFET.

In a grain size measuring method, as shown in FIG. 8A, the surface ofthe abrasive is observed, for example, with a 100 μm/side field ofvision using a Scanning Electron Microscope and, as shown in FIG. 8B,lengths in an X-direction and a Y-direction are measured for eachabrasive grain confirmed in the field and then the average value thereofis calculated, so that the abrasive grain size can be calculated. Sincethe abrasive grains are directed randomly, all x and y values ofmeasured lengths are averaged, so that the abrasive grain size isobtained. Further, the x value is a value resulted from lengthmeasurement of the width in the X-direction of each abrasive grain inthe field and the y value is a value resulted from length measurement ofthe width in the Y-direction of each abrasive grain in the field. Here,the abrasive grain size means an average abrasive grain size in theabrasive with respect to diamond, a crystal made of alumina, boron andnitrogen, and the like, that may be contained in that abrasive. Notethat the field may be changed as appropriate according to the sizes ofthe abrasive grains and the degree of concentration thereof.

As you can see by looking at that graph, when the average abrasive grainsize of the abrasive used at the last grinding is set to 1 μm or more,it is possible to reduce the ON-resistance of the SiC-MOSFET at the timeof energization because the SiC substrate and the drain electrode can beelectrically bonded together due to increase in the unevenness of thesurface, introduction of a fractured layer by mechanical grinding of theSiC substrate, and the like. Note that, at the time of measurement, sucha value is determined as the ON-resistance of the semiconductor device,that is based on a voltage drop occurring when a voltage of 15 V isapplied between the gate and the source and the semiconductor device isenergized up to the rated current, and that is obtained when “a voltagedrop value of the semiconductor device” is normalized by a voltage dropvalue thereof obtained when the average abrasive grain size of theabrasive is 1 μm.

Further, in FIG. 7B, there is shown a relationship at the time thethinning processing shown in FIG. 5, FIG. 6A and FIG. 6B is executed,between an average abrasive grain size of the abrasive used at the lastgrinding and a ball transverse rupture strength of the cell structure101 of the obtained SiC-MOSFET that is normalized according to the balltransverse test defined by JIS G 0202. As you can see by looking at thatgraph, when the average abrasive grain size of the abrasive used at thelast grinding is set to 5 μm or less, it becomes possible to reduce thegrinding damage due to grinding processing to thereby achieve sufficienttransverse rupture strength for the SiC-MOSFET. Thus, when the multiplesemiconductor devices formed on the wafer are segmentalized into therespective semiconductor devices, it is possible to reduce chipping orcracking of the segmentalized semiconductor device at its end portionoccurring when an external force due to dicing is applied thereto; andwhen the semiconductor device (chip) is mounted in a semiconductormodule, it is possible to reduce cracking of the semiconductor device(chip) caused by a stress applied to the chip due to a difference inlinear expansion coefficient between a solder or like foreign materialand SiC. This makes it possible to improve the yield significantly.

Accordingly, when the average abrasive gain size is set to not less than1 μm and not more than 5 μm according to FIG. 7A and FIG. 7B, it ispossible not only to establish excellent bonding but also to achieve aSiC semiconductor device having a high mechanical strength with areduced energization loss.

As described above, the semiconductor device manufacturing methodaccording to Embodiment 1 of the application comprises: a step offorming the first drift layer 2 made of n-type SiC on the front surfaceof the n-type SiC substrate 1; a step of forming the second drift layer3 as a second-layered epitaxial film on the surface of the first driftlayer 2; a step of forming the pair of p-type base regions 4 on thesurface of the second drift layer 3, then forming the n-type sourceregion 5 partly on each of surfaces of the p-type base regions 4, andthereafter forming the base regions 4 a and source regions 5 a that areelectrically activated; a step of forming the gate electrode 7 thatextends astride between the pair of base regions 4 a and between thepair of source regions 5 a, with the gate insulating film 6 sandwichedbetween that electrode and these regions; a step of grinding the backsurface of the SiC substrate 1 and forming the SiC substrate-to-drainelectrode bonding region 9, by using an abrasive whose average grainsize is within the specified range; a step of forming the drainelectrode 10 on the SiC substrate-to-drain electrode bonding region 9; astep of electrically connecting the drain electrode 10 to the SiCsubstrate-to-drain electrode bonding region 9; and a step of forming thesource electrode 8 on the front-surface side after the drain electrode10 was thus electrically connected. Thus, it becomes possible toestablish excellent electrical bonding between the SiC substrate and thedrain electrode and to reduce the grinding damage due to grindingprocessing, to thereby achieve a SiC semiconductor device having a highmechanical strength with a reduced energization loss.

Embodiment 2

In Embodiment 1, a step is shown in which the source electrode is formedafter the formation of the drain electrode, whereas, in Embodiment 2,such a case will be described where the source electrode is formedbefore the formation of the drain electrode.

FIG. 9 is a sectional view showing a cell structure of a main part of asemiconductor device manufactured by the semiconductor devicemanufacturing method according to Embodiment 2 of the application. Asshown in FIG. 9, in a cell structure 102, on the back-surface side ofthe SiC substrate 1, a drain electrode (1) 29 for getting bonding withthe SiC substrate through the drain electrode bonding region 9 is formedon the SiC substrate-to-drain electrode bonding region 9 that is formedon the back-surface side of the SiC substrate 1 having been ground awayto the extent corresponding to a specified thickness thereof, and adrain electrode (2) 30 as a second main electrode is further formed onthe surface of the drain electrode (1) 29 as a first main electrode. Theother configuration is the same as that in the semiconductor device ofEmbodiment 1, so that its description is omitted here.

The semiconductor device manufacturing method according to Embodiment 2of the application will be described based on FIG. 10. FIG. 10 is aflowchart showing manufacturing steps in the semiconductor devicemanufacturing method according to Embodiment 2 of the application. FIG.11A, FIG. 11B and FIG. 11C are sectional views of a cell at respectivesemiconductor-device manufacturing steps according to Embodiment 2.

First of all, with respect to the preparatory processing for the SiCsubstrate through the interlayer insulating film forming processing, thesame steps as Step S301 to Step S307 in Embodiment 1 are taken.

Then, in source electrode forming processing (Step S1008), after thefront-surface side of the SiC semiconductor device was cleanedbeforehand by using, on the front-surface side of the wafer, an aqueoussolution containing hydrofluoric acid or an aqueous solution containingammonia and hydrogen peroxide water, a metal barrier made of titanium ora titanium compound such as titanium nitride (TiN) or the like is formedon the source regions 5 a and the interlayer insulating film 6 b, andthereafter, a film of the source electrode 8 made of aluminum, analuminum alloy of aluminum and silicon, nickel or the like is depositedand subjected to patterning, so that a cell structure on thefront-surface side is completed as shown in FIG. 11A. Here, the materialto be used for the source electrode 8 may be selected as appropriateaccording to a bonding method on the front-surface side, such as wiring,soldering or the like.

Subsequently, in SiC substrate thinning processing (Step S1009), asshown in FIG. 11B, thinning of the substrate is performed from theback-surface side in such a manner that machining is applied using agrinding abrasive made up of aluminum abrasive grains or diamondabrasive grains, to the back surface of the SiC substrate 1, thereby tomake the SiC substrate 1 thin and further to form, on the back-surfaceside of the substrate, the region for getting bonding between the SiCsubstrate and the drain electrode, in which unevenness or crystal straindue to grinding is introduced for getting bonding with a back-surfaceside electrode.

For the thinning processing, infeed grinding may be applied in which, asshown in FIG. 5, FIG. 6A and FIG. 6B, the grinding wheel 26 is movedcloser at a constant speed onto the wafer that is rotating on thegrinding stage 18, 19.

Instead, creep feed grinding may be carefully applied in which aworkpiece is moved in a fixed direction against an abrasive that isrotating.

Then, in drain electrode (1) forming processing (Step S1010), after theexecution of cleaning on the back-surface side of the wafer by using anaqueous solution containing hydrofluoric acid or an aqueous solutioncontaining ammonia and hydrogen peroxide water, the drain electrode (1)29 is formed. Specifically, a 10 to 200 nm nickel film is formed on theback-surface side by sputtering, evaporation or the like.

Subsequently, in SiC substrate-to-drain electrode bonding processing(Step S1011), as shown in FIG. 11C, electrical bonding between the SiCsubstrate and the drain electrode is carried out in such a manner thatthe back-surface side of the semiconductor substrate is heated by laserusing a sold-state laser annealing system SWA-90GD from Sumitomo HeavyIndustries, Ltd, Japan, or the like. Irradiation by laser makes itpossible to heat it without affecting the source electrode 8 formed onthe front surface.

With respect to the irradiation energy of laser, when the energy densityis set to 0.5 to 3.0 J/cm², it is possible not only to establishexcellent electrical bonding between the SiC substrate and the electrodebut also to suppress occurrence of cracking in the SiC substrate.

As is shown by FIG. 12A, it is found that when the irradiation energy oflaser is set to 0.5 J/cm² or more, a sufficient temperature of 500° C.or more is applied to the SiC substrate and the nickel film, so that SiCand nickel forms an alloy.

In FIG. 12B, a relationship between an average abrasive grain size ofthe abrasive at the thinning processing and a cracking ratio of the SiCsubstrate at the heating of that substrate by the laser irradiation, isshown for each of cases where the density of the irradiation energy oflaser is varied. According to FIG. 12B, it is shown that, when theirradiation energy of laser becomes 3.1 J/cm² or more, the temperaturegradient at a portion irradiated by laser becomes steep, so thatcracking occurs in the SiC substrate. Thus, when it is set to less than3.1 J/cm², namely, set to 3.0 J/cm² or less, it becomes possible toreduce the cracking ratio at the laser irradiation, so that theprocessing loss during manufacturing of the device can be reducedremarkably.

It is noted that, according to Embodiment 2, a relationship like thatshown in FIG. 7B between the average abrasive grain size and the balltransverse rupture strength has also been obtained and thus, when thedensity of the irradiation energy of laser per one irradiation is set to0.5 to 3.0 J/cm², it is possible by the laser irradiation to recover adefective layer due to mechanical grinding and introduced at thethinning processing, while promoting the alloying of SiC and nickel.With respect to the recovery effect, the higher the energy densitybecomes, the more the effect is created, and a density of theirradiation energy that is more than 1.5 J/cm² is preferable. Note thatsuch a tendency that is similar to in the result in FIG. 12A and FIG.12B are shown in cases where the thickness of the SiC substrate is notless than 50 μm and not more than 150 μm.

It is noted that, for bonding the SiC substrate with the nickel film asthe drain electrode (1), it is preferable that the thickness of thenickel film be 10 nm or more in order to surely form an alloy of SiC andnickel, and it is desirable that it be 200 nm or less in order tosuppress cracking of the wafer due to thermal shock at the laserirradiation for achieving alloying. Further, the wavelength of the laserannealing system may be carefully changed from within the range of 300to 600 nm.

In the case where the grinding is performed using an abrasive having anaverage abrasive grain size of not less than 1 μm and not more than 5μm, the nickel film is formed on the surface, and the density of theirradiation energy is set to not less than 1.5 and not more than 3.0J/cm², it is more preferable that the thickness of nickel be not lessthan 20 nm and not more than 80 nm. If it is thinner than 20 nm, thismay affect the electrical property, such as causing, for example,increase in conduction resistance between the SiC substrate and thenickel film, and if it is thicker than 80 nm, this may degrade therecovery effect of the defects by the irradiation energy of laser.

Further, in view of the transverse rupture strength, in the case wherethe grinding is performed using an abrasive having an average abrasivegrain size of not less than 1 μm and not more than 5 μm, the nickel filmis formed on the surface, and the density of the irradiation energy isset to not less than 1.5 and not more than 3.0 J/cm², the effect will bemore remarkable when the grinding amount is 250 μm or more. When thegrinding amount is large, it is expected that many defective layers areprovided and thus the recovery effect is significant.

Lastly, in drain-electrode (2) forming processing (Step S1012), on thesurface subjected to grinding processing on the back-surface side, thedrain electrode (2) having a thickness of 10 to 50 times (for example,600 nm) that of the laser-irradiated drain electrode (1), is formedusing the same material of the drain electrode (1), for example, anickel film is formed by sputtering, evaporation or the like, so thatthe cell structure 102 of the SiC semiconductor device shown in FIG. 9is completed.

As described above, according to the relationship between theON-resistance characteristic and the strength, the thickness of thedrain electrode (1) is preferably not less than 10 nm and not more than200 nm, and more preferably not less than 20 nm and not more than 80 nm.If this is the case, the drain electrode (1) functions as a contact tothe SiC substrate; however, in consideration of a contact to themounting board, the thickness of the drain electrode (2) is preferably10 to 50 times that of the drain electrode (1). If it is less than 10times, the drain electrode (2) may be diffused, at the time it is bondedwith the mounting board, into a bonding material such as a solder, tothereby disappear. On the other hand, if it is more than 50 times,warpage due to the stress from the drain electrode (2) may occur in theSiC semiconductor device and thus a gap may emerge between the mountingboard and the drain electrode (2), so that the conduction loss mayincrease due to reduction in their contact area.

When a nickel layer is formed in two steps in the above manner, it ispossible to achieve bonding with the mounting board by using the drainelectrode (2) while forming an assured contact to the SiC substratethrough laser irradiation by using the drain electrode (1), and thusthis case is preferable.

It is noted that, when nickel is used for the source electrode 8 and thedrain electrode (2) 30, the bonding state at the time the chip is bondedwill be degraded because the wettability between the solder alloy andnickel becomes poor due to oxidation of the surface, so that, on thesurface of nickel, a metal that is outwardly less reactive than that,such as gold, silver or the like, may be used as a protective film.

In the above manner, the laser irradiation is used for SiCsubstrate-to-drain electrode bonding after the formation of the sourceelectrode as shown in FIG. 10, so that, as compared with the case wherethe source electrode is formed after the thinning of the SiC substrateas shown in FIG. 3, it becomes unnecessary to perform in the thinnedstate, film deposition of the source electrode and its patterning byphotolithography, plasma etching and/or wet etching. This makes itpossible to reduce chipping or cracking of the wafer due to handling ofthe wafer during processing, so that the productivity can be improved.

Namely, at the time of thinning the SiC substrate by grinding; formingthe nickel electrode and then executing the laser irradiation; after theformation of the nickel electrode, forming a metal such as gold, silveror the like as a protective film on that surface; and mounting the waferon a dicing ring and segmentalizing it by dicing; it is possible toreduce the cracking ratio of the wafer after the laser irradiation andthe cracking ratio of the chip at the dicing when the average abrasivegrain size is not less than 1 μm and not more than 5 μm, and the energydensity is not less than 1.5 and not more than 3.0 J/cm².

Further, in order to suppress nickel from being consumed due to solderbonding at the time of assembly, it is allowable for the sourceelectrode and the drain electrode to perform depositing a thick nickelfilm exceeding 3 μm by nickel electroplating or nickel-phosphorouselectroless plating. Further, in the drain electrode (1) formingprocessing (Step S1010) that is required for the bonding with the SiCsubstrate and in the drain electrode (2) forming processing (Step S1012)that is required for the bonding between the SiC semiconductor deviceand the mounting board for that device, it becomes possible to select afavorable film-deposition condition for each of them.

With respect also to the thus-obtained cell structure 102, a similarresult to that in Embodiment 1 has been achieved regarding theON-resistance that is reflective of the energization loss and the balltransverse rupture strength that is reflective of the mechanicalstrength. In Embodiment 2, for making the cell structure 102 thin,thinning processing has been executed also by the infeed grinding systemusing diamond abrasive grains. The relationship between the averageabrasive grain size and the ON-resistance is similar to that in FIG. 7Aand, when the average abrasive grain size of the abrasive used at thelast grinding is set to 1 μm or more, a sufficient contact area betweenthe SiC substrate and the drain electrode is ensured due to increase inthe unevenness of the surface, so that the ON-resistance of theSiC-MOSFET at the time of energization could be reduced. On the otherhand, the ball transverse rupture strength is also similar to that inFIG. 7B, and when the average abrasive grain size of the abrasive usedat the last grinding is set to 5 μm or less, the unevenness of thesurface decreases, so that a sufficient transverse rupture strengthcould be achieved for the SiC-MOSFET.

Accordingly, even in Embodiment 2, when the average abrasive gain sizeis set to not less than 1 μm and not more than 5 μm according to FIG. 7Aand FIG. 7B, it is possible not only to establish excellent bonding butalso to achieve a SiC semiconductor device having a high mechanicalstrength with a reduced loss by electrical bonding and by energization.

As described above, according to the semiconductor device manufacturingmethod according to Embodiment 2 of the application, after theformations of up to the source electrode 8 on the front-surface side ofthe SiC substrate 1, the back surface of the SiC substrate is ground andthe SiC substrate-to-drain electrode bonding region 9 is formed, byusing an abrasive whose average grain size is within a specified range,and after the drain electrode (1) 29 formed thinly on the SiCsubstrate-to-drain electrode bonding region 9 was electrically bondedthereto by the laser irradiation, the drain electrode (2) 30 is furtherformed on the laser-irradiated drain electrode (1) 29. Thus, it ispossible not only to establish excellent bonding and to achieve a SiCsemiconductor device having a high mechanical strength with a reducedenergization loss, similarly to Embodiment 1, but also to form thesource electrode before grinding of the SiC substrate. This reduces thenumber of steps in the thinned state, and thus a film quality of drainelectrode that is favorable for each of the SiC substrate and themounting board can be selected while reducing the wafer breakage rateduring processing.

It is noted that, in Embodiment 2, the source electrode is formed beforegrinding of the SiC substrate; however, similarly to Embodiment 1, thesource electrode 8 may be formed after the formation of thelaser-irradiated drain electrode (1) 29 or after the formation of thedrain electrode (2) 30. Even in this case, an effect similar to that inEmbodiment 1 can be achieved.

Further, in the above embodiments, although a SiC substrate is used asthe semiconductor substrate, this is not limitative. As its material,silicon (Si) or another material, such as gallium nitride (GaN), diamondor the like serving as a wide bandgap semiconductor material, is used.

It should be noted that unlimited combination of the respectiveembodiments and an appropriate modification/omission in the embodimentsmay be made in the present application without departing from the scopeof the application.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

1: SiC substrate, 2: first drift layer, 3: second drift layer, 4, 4 a:base regions, 5, 5 a: source regions, 6: gate insulating film, 6 a:insulating film, 6 b: interlayer insulating film, 7: gate electrode, 10drain electrode, 29: drain electrode (1), 30: drain electrode (2).

The invention claimed is:
 1. A method for manufacturing a semiconductordevice, comprising: a first grinding step of grinding, using anabrasive, a back-surface side of a SiC substrate having an active regionformed on a front-surface side thereof; a second grinding step ofgrinding a back-surface side of the SiC substrate provided after thefirst grinding step, using an abrasive having an average abrasive grainsize of not less than 1 μm and not more than 5 μm; a step of depositinga film of a first main electrode on an electrode bonding region formedby the second grinding step; a step of electrically connecting the firstmain electrode with the electrode bonding region by using laserirradiation; and a step of depositing a film of a second main electrodeon the first main electrode subjected to the laser irradiation, whereina grinding amount of the SiC substrate for grinding the back-surfaceside by the first grinding step and the second grinding step is 250 μmor more; and wherein a thickness of the SiC substrate becomes not lessthan 50 μm and not more than 150 μm.
 2. The method for manufacturing asemiconductor device of claim 1, wherein the second grinding step is alast grinding step.
 3. The method for manufacturing a semiconductordevice according to claim 2, wherein irradiation energy of a laser thatis used at the time of electrically connecting the first main electrodewith the electrode bonding region by using the laser irradiation, is notless than 0.5 J/cm² and not more than 3.0 J/cm².
 4. The method formanufacturing a semiconductor device according to claim 3, wherein thefirst main electrode has a thickness of not less than 10 nm and not morethan 200 nm.
 5. The method for manufacturing a semiconductor deviceaccording to claim 4, wherein the second main electrode has a thicknessthat is not less than 10 times and not more than 50 times that of thefirst main electrode.
 6. The method for manufacturing a semiconductordevice according to claim 3, wherein the second main electrode has athickness that is not less than 10 times and not more than 50 times thatof the first main electrode.
 7. The method for manufacturing asemiconductor device according to claim 3, wherein the first mainelectrode and the second main electrode are made of nickel.
 8. Themethod for manufacturing a semiconductor device according to claim 2,wherein the first main electrode has a thickness of not less than 10 nmand not more than 200 nm.
 9. The method for manufacturing asemiconductor device according to claim 8, wherein the second mainelectrode has a thickness that is not less than 10 times and not morethan 50 times that of the first main electrode.
 10. The method formanufacturing a semiconductor device according to claim 2, wherein thesecond main electrode has a thickness that is not less than 10 times andnot more than 50 times that of the first main electrode.
 11. The methodfor manufacturing a semiconductor device according to claim 2, whereinthe first main electrode and the second main electrode are made ofnickel.
 12. The method for manufacturing a semiconductor deviceaccording to claim 1, wherein irradiation energy of a laser that is usedat the time of electrically connecting the first main electrode with theelectrode bonding region by using the laser irradiation, is not lessthan 0.5 J/cm² and not more than 3.0 J/cm².
 13. The method formanufacturing a semiconductor device according to claim 12, wherein thefirst main electrode has a thickness of not less than 10 nm and not morethan 200 nm.
 14. The method for manufacturing a semiconductor deviceaccording to claim 13, wherein the second main electrode has a thicknessthat is not less than 10 times and not more than 50 times that of thefirst main electrode.
 15. The method for manufacturing a semiconductordevice according to claim 12, wherein the second main electrode has athickness that is not less than 10 times and not more than 50 times thatof the first main electrode.
 16. The method for manufacturing asemiconductor device according to claim 12, wherein the first mainelectrode and the second main electrode are made of nickel.
 17. Themethod for manufacturing a semiconductor device according to claim 1,wherein the first main electrode has a thickness of not less than 10 nmand not more than 200 nm.
 18. The method for manufacturing asemiconductor device according to claim 17, wherein the second mainelectrode has a thickness that is not less than 10 times and not morethan 50 times that of the first main electrode.
 19. The method formanufacturing a semiconductor device according to claim 1, wherein thesecond main electrode has a thickness that is not less than 10 times andnot more than 50 times that of the first main electrode.
 20. The methodfor manufacturing a semiconductor device according to claim 1, whereinthe first main electrode and the second main electrode are made ofnickel.